This invention generally concerns offset printing machines and, more particularly, an offset duplicator equipped with an automatic device for applying and removing a master, which can be automatically controlled by a control device capable of switching on and off the various functions of the machine in succession according to a pre-set program, and with a treating device having a pass-through dip tank for converting the master as well as conveyor rolls for moving the master away from the dip tank.
In order to use such offset duplicator with masters, known as zinc oxide masters which are imaged on electrostatic master imagers or copiers, it is necessary to equip the machine with a treating device known as a converter apparatus. Such treating devices are used for the chemical pre-treatment of the zinc oxide masters to render the unimaged areas of the master moisture-receptive.
Most of the known office offset printing machines are equipped with so-called etching or pre-moistening mechanisms in which etching fluid is applied to the surface of direct image and other masters, but not to zinc oxide masters which require conversion. Direct image masters are normally treated after the master is attached to the master cylinder, by means of a saturated roller or a non-rotatable applicator member having a saturated absorptive coating. However, because zinc oxide masters require a conversion rather than an etching treatment, in order to properly treat the master it is not normally converted on the master cylinder. Only after such treatment of the master can the duplicator be operated further according to a pre-set program.
It is known that the most satisfactory conversion method comprises passing the master to be treated through a conversion fluid contained in a pass-through dip tank, by advancing the master through the tank at a proper transport speed prior to its being attached to the master cylinder.
In one known machine which utilizes this method, a duplicator and an electrostatic master imaging apparatus are combined into one structural and functional unit as shown, for example, in U.S. Pat. No. 3,426,678. In the device of the patent there is situated in the transport path of the master, following a developer station in the transport path, a conversion bath through which the zinc oxide master must pass. After leaving the conversion bath the master arrives first at a waiting station which is provided with a pair of conveyor rolls positioned directly adjacent the master cylinder to which the master is to be attached. The conveyor rolls, which in their normal position are lifted or separated from each other, are driven by the master cylinder at the peripheral speed of, and independently of any conveyor part of the copying or conversion apparatus. Since the master speed through the conversion bath is relatively low to allow time for the required chemical reaction, while the speed of the master cylinder is normally high to provide high productivity, the waiting station is required. However, such a waiting station requires a suitable support at ready position on which the master to be attached to the master cylinder can repose while it is waiting to be attached to the cylinder. This waiting station extends the overall size of the machine since it requires a surface area sufficiently large to accommodate the master.
In known offset machines which provide a treating device with a pass-through dip tank, such waiting stations for the converted master are required, as mentioned supra, because the converting speed, i.e., a speed of about 3.5 meters per minute at which the master must be passed the dip tank to be converted properly, does not correspond with the printing speed which is about 50 meters per minute. However, because such a master feeding device having a treating tank includes in its lengthwise dimension a stack of master from which individual masters are advanced, the treating tank with squeeze rolls and the waiting station for the converted master, the overall size of the machine is large and bulky and does not readily provide a compact office offset duplicator.